This invention pertains generally to the field of nanofabrication techniques and particularly to nanofabrication carried out utilizing diblock copolymers.
Significant challenges are encountered in the fabrication of nanostructures, particularly structures at a length scale of 10 nm to 50 nm. It is possible to fabricate isolated or semi-dense structures at this scale with an advanced lithographic technique such as electron beam lithography, but the exposure tools are extremely expensive and optimization of photo-resist processing is non-trivial and may not be amenable to strict control of dimensions or roughness.
New processes and novel materials are required to make nanofabrication easier, cheaper, and more versatile. Block copolymers are interesting materials for use in nanofabrication because they microphase separate to form ordered, chemically distinct domains with dimensions of 10""s of nm. The size and shape of these domains can be controlled by manipulating the molecular weight and composition of the copolymer. Additionally, the interfaces between these domains have widths on the order of 1-5 nm and can be controlled by changing the chemical composition of the blocks of the copolymers. An advantage of using block copolymer systems as templates is that linewidth, tolerances and margins, and line edge roughness are dictated by thermodynamics (molecular weight, the Flory-Huggins interaction parameters "khgr" between the blocks of the copolymer). It is unclear whether standard resist processing, where performance depends on control of kinetic processes, will be applicable at the scale of 10xe2x80x2s of nm.
The feasibility of using thin films of block copolymers as templates was demonstrated previously by Chaikin and Register, et al., Science 276, 1401 (1997). Dense arrays of dots and holes with dimensions of 20 nm were transferred from a thin film of poly(styrene-b-isoprene) to silicon nitride substrates. The perfection of ordering of domains extend over grain sizes of approximately 1 xcexcm2. For many applications, macroscopic orientation of the copolymer domains over areas as large as several cm2 and registration of the domains with the substrate will be required. Thin films with macroscopically ordered domains are envisioned as having potential in several applications including nanowires, magnetic storage media, quantum devices, and photonic crystals. Strategies for inducing macroscopic orientation of copolymer domains in thin films have included: (1) the use of electric fields to orient cylindrical domains in asymmetric diblock copolymer films both parallel to the film along electric field lines and perpendicular to the film in hexagonal arrays, (2) the use of miscut silicon wafers as substrates to align thickness induced morphologies along the corrugations of the substrate, (3) the use of miscut silicon wafers with obliquely deposited Au stripes to promote alternating wetting of the blocks on the alternating Si and Au stripes and perpendicular orientation of lamellar domains, and (4) the use of sidewall constraints to induce long range ordering of spherical domains in asymmetric diblock copolymers.
One approach to inducing macroscopic orientation of the domains of block copolymers combines advanced lithographic techniques and the self-assembly of the block copolymer film. Organic imaging layers are patterned using lithographic tools, e.g., proximity x-ray lithography with a mask and extreme ultraviolet (EUV) interferometric lithography. Regions of the imaging layer that are exposed to radiation or electrons undergo a chemical transformation that alters the surface chemistry of the imaging layer. A thin film of a symmetric diblock copolymer is then deposited on the patterned imaging layer and annealed above the glass transition temperature of the blocks of the copolymer. During annealing, the lamellar domains of the copolymer film self-assemble such that adjacent regions of the chemically patterned surface are wet by the different blocks of the copolymer. The lamellae orient perpendicular to the plane of the film and amplify the surface pattern. After annealing, selective removal of one of the blocks results in a nanopatterned template that can be used for additive or subtractive processes for nanofabrication. This strategy has the advantages of achieving macroscopic orientation of the lamellar domains using parallel exposure tools and registration of the patterned film with the substrate. See, Richard D. Peters, et al., xe2x80x9cUsing Self-Assembled Monolayers Exposed to X-Rays to Control the Wetting Behavior of Thin Films of Diblock Copolymers,xe2x80x9d Langmuir, Vol. 16, 2000 (published on web Apr. 7, 2000), pp. 4625-4631; Qiang Wang, et al., xe2x80x9cSymmetric Diblock Copolymer Thin Films Confined Between Homogenous and Patterned Surfaces: Simulations and Theory,xe2x80x9d Journal of Chemical Physics, Vol. 112, No. 22, Jun. 8, 2000, pp. 9996-10010; Tae K. Kim, et al., xe2x80x9cChemical Modification of Self-Assembled Monolayers by Exposure to Soft X-Ray in Air,xe2x80x9d J. Phys. Chem. B., Vol. 104, 2000 (published on web Jul. 18, 2000), pp. 7403-7410.
In the present invention, advanced interferometric lithography is combined with self-assembled block copolymer systems to provide nanofabricated structures. Interferometry is used to pattern substrates with regions of different chemical functionality in spatial arrangements commensurate with the characteristic dimensions of the domain structure of the polymer. Upon ordering, the morphology of a block copolymer layer on the surface of the substrate is guided toward the desired long-range orientation, amplifying the pattern on the surface. The block copolymers can be synthesized for guided self-assembly and either are functional as formed or can be functionalized after microstructures are formed.
In forming the copolymer microstructures in accordance with the invention, a substrate is provided with an imaging layer thereon that will respond to exposure to selected wavelengths to change the wettability of the exposed material of the imaging layer to the components of a selected block copolymer. The imaging layer is then exposed to two or more beams of radiation within the selected wavelengths to form interference patterns at the imaging layer to change the wettability of the imaging layer in accordance with the interference patterns. Preferably, the interference pattern in the imaging layer has a period substantially equal to, and preferably within 20% of, the bulk lamellar period L0 of the selected copolymer. For interference patterns having periods in the range of 10-100 nm, the exposing radiation preferably is at selected wavelengths in the extreme ultraviolet or shorter. A layer of the selected copolymer is then deposited onto the exposed imaging layer, and the copolymer layer is annealed to separate the components of the copolymer in accordance with the pattern of wettability in the underlying imaging layer to replicate the pattern of the imaging layer in the copolymer layer. For example, one of the beams may be provided directly from the source onto the imaging layer and the other beam may be provided by reflecting a portion of the beam from the same source with a Lloyd""s mirror onto the imaging layer at an angle to the beam that is directly incident on the imaging layer. Where two such beams are utilized, the resulting pattern in the imaging layer defined by the interference pattern is a periodic pattern of alternating stripes which differ in wettability with respect to one of the components of the copolymer. The resulting microstructure in the annealed copolymer layer can comprise corresponding alternating stripes of the two components of the copolymer that are separated in accordance with the regions of greater or lesser wettability. Other interferometric processes and instruments may also be utilized.
A third beam may be provided to the imaging layer by reflecting a portion of the beam from the same source with another Lloyd""s mirror onto the imaging layer at an angle to the beam that is directly incident on the imaging layer. The two Lloyd""s mirrors are positioned at an angle to each other to provide interference patterns at the imaging layer that are at an angle to each other. In this manner, an array of separated regions can be defined in the imaging layer in which the separated regions have a higher or lower wettability with respect to one of the components of the copolymer. Deposit of the copolymer layer and annealing of the copolymer layer results in regions of one of the components of the copolymer separated primarily by components of the other copolymer in a pattern which corresponds to the underlying pattern in the imaging layer. The imaging layer may be formed of materials such as self-assembled monolayers such as alkylsiloxanes. Various block copolymers may be utilized, one example of which is a copolymer of polystyrene and poly(methyl methacrylate).
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.